1. Field of the Invention
The present invention relates to the field of magnetic metal or metal carbide compounds and carbon-coated magnetic metal or metal carbide compounds. Particularly, the present invention relates to the field of carbon-coated magnetic metal or metal carbide nanoparticles and the methods for preparing the same. Nanoparticles include crystalline or amorphous particles 5 to 60 nanometers in diameter and nanotubes up to 1 centimeter long and 5 to 60 nanometers in diameter.
2. Description of the Prior Art
Small magnetic particles have many applications. Such particles are used as toner in xerography, in ferrofluid vacuum seals, in nuclear magnetic resonance imaging as contrast agents, and in magnetic data storage. These magnetic particles are typically micron-sized in diameter or larger. The large size of these particles renders them less than satisfactory for several specialized applications.
If the magnetic particles were smaller, better resolution would be achieved in xerographic applications. In ferrofluid applications, the enhanced solubility due to carbon coating provided by smaller particles may be advantageous. In magnetic data storage, resolution may be enhanced by using smaller particles. Consequently, there is a potential need for sub-micron sized magnetic metal carbon-coated particles and a method for producing bulk amounts of these particles in a high yield process.
Recently, there has been increased investigation regarding the Kratschmer-Huffman carbon arc method of preparing fullerenes, or small hollow carbon clusters. These fullerenes are typically in the order of 1 nm in diameter. Recently, it has further been discovered that these hollow carbon clusters can be filled with metal ions. This can be accomplished by drilling out the graphite rods and packing them with a mixture of metal oxide powder and graphite cement before generating the soot by the carbon arc. Rodney S. Ruoff, Donald C. Lorents, Bryan Chan, Ripudaman Malhotra, and Shekhar Subramoney, Science, Vol. 259, p. 346 (1993) discussed the production of 20-40 nm diameter carbon-coated lanthanum carbide nanocrystallites by this method. Similar results were reported by Masato Tomita, Yahachi Saito and Takayoshi Hayashi in Jpn. J.Appl. Phys., Vol. 32, p. 280 (1993).
The carbon arc method of preparing lanthanum carbide nanocrystallites discussed above generates fullerenes and graphitic soot in addition to the ianthanum carbide nanocrystailites. In order Lo be useful, a means of separating the nanocrystallites is essential. So far, no chemical methods have been found to be successful in separating macroscopic amounts of nanoparticles from graphitic soot and fullerenes. Such separation processes are rendered extremely important when the yields achieved for the nanoparticles is in the order of ten percent or less of the soot. Accordingly, there is a need for a method to separate carbon-coated metal nanoparticles from graphitic soot.